1. Field of the Invention
This application claims priority under 35 U.S.C. § 119 from French Patent Application No. 04 50 575, filed on Mar. 23, 2004, in the French Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
The invention concerns the field of reflectarray antennas, and more particularly the phase shifter modules that equip such antennas.
2. Description of the Related Art
Reflectarray antennas are one of the two main array antenna families, the other family being phased array antennas. These array antennas are particularly advantageous as they can be reconfigured, for example to allow switching from one coverage area (or “spot”) to another.
A reflectarray antenna is made up of radiating elements designed to intercept, with minimum losses, waves consisting of signals to be transmitted, emitted by a primary source, in order to reflect them in a chosen direction, known as the pointing direction. To allow the reconfigurability of the antenna diagram, each radiating element is equipped with a phase control device with which it forms a passive or active phase shifter module.
“Phase shifter module” is understood to mean both radiating cavity and slot structures and radiating patch resonant planar structures.
The invention more particularly concerns linear polarization active phase shifter modules. These usually consist of a phase shifter module with a switch made up of diodes (usually the PIN type), MESFETs, varactors, or mechanical control devices (such as, for example, a motor designed to move a dielectric rod).
Switch-operated phase shifter modules consume a large amount of energy and are subject to significant losses and heating. Mechanical control phase shifter modules are complicated to implement, in particular in the case of large arrays, and consume a lot of energy. In both cases, the disadvantages entailed by the phase control techniques used limit the applications of phase shifter modules, particularly in the aerospace field, and more specifically in observation platforms, such as satellites, for example.
The object of the invention is therefore to improve the situation in the case of linear polarization active phase shifter module reflectarray antennas.
To this end, it proposes a phase shifter module with a characteristic resonant length that, in at least one selected place, has an MEMS (Micro ElectroMechnical System) device able to be placed in at least two different states respectively permitting and prohibiting the establishing of a short-circuit intended to vary the characteristic resonant length, in order to vary the phase shifting of the waves to be reflected that present at least one linear polarization.
Each MEMS device may, for example, consist of a flexible conducting bridge whose states are controlled by two control electrodes that are placed roughly on top of each other, one of which is comprised of the bridge. Alternatively, each MEMS device may consist of a suspended flexible conducting beam (or cantilever) whose states are controlled by a control electrode placed below its suspended section.
In one family of embodiments, the module may have a resonant planar structure consisting of at least one rectangular upper patch placed roughly parallel to a lower ground plane, at a selected distance, the lower ground plane defining at least one conducting “wafer”, that may be rectangular, for example, completely surrounded by a non-conducting zone, placed below the upper patch and of smaller dimensions. In this case, the module has at least one metallic bushing connecting the upper patch to the wafer and the MEMS device is placed in the non-conducting zone, in order to establish, in one of its states, a link between the wafer and the rest of the ground plane to control the resonant length of the upper patch.
The lower ground plane may possibly define at least two wafers (that may be rectangular, for example) completely surrounded by a non-conducting zone, placed below the upper patch and of smaller dimensions. In this case, the module has at least two metallic bushings respectively connecting the upper patch to one of the wafers, and at least two MEMS devices each placed in one of the non-conducting zones, in order to establish links between at least one of the wafers and the rest of the ground plane, allowing the defining of at least three upper patch resonant lengths that differ according to their states.
As a variant of this family of embodiments, the module may consist of an upper ground plane with at least one radiating slot, equipped with an MEMS device controlling its characteristic resonant length, a lower ground plane and metallic bushings connecting the lower ground plane to peripheral sections of the upper ground plane in order to define a resonant cavity. For example, the upper ground plane may have at least two radiating slots, each equipped with a single MEMS device controlling their characteristic resonant length. Each MEMS device may thus preferably be placed roughly in the middle of a radiating slot. Furthermore, the slots are preferably roughly parallel to each other and may have slightly different lengths. They may also be curved, however, so that together they form an annular slot short-circuited at two roughly opposite points.
Alternatively, the upper ground plane may have one radiating slot, equipped with at least two MEMS devices allowing the defining of at least three resonant lengths that differ according to their states.
Moreover, the upper ground plane may possibly have at least one rectangular radiating slot with large sides parallel to a first direction, and at least one other rectangular radiating slot with large sides parallel to a second direction perpendicular to the first, in order to allow a double linear polarization.
In another family of embodiments, the module may consist of a resonant planar structure comprising an upper patch placed roughly parallel to a lower ground plane, at a selected distance. In this case, the patch has at least one slot equipped with at least one MEMS device controlling its characteristic resonant length.
The module may thus have a single slot (of a half-wave length) equipped with at least two MEMS devices, allowing the defining of at least three resonant lengths that differ according to their states. As an alternative, the upper patch may be roughly square and the module may have at least a first and second rectangular slot (of a quarter-wave length) placed roughly opposite each other, coming out onto two non-radiating opposite sides of the square, each being equipped with at least two MEMS devices, allowing the defining of at least three resonant lengths that differ according to their states. In this last case, the module may also have at least a third and fourth rectangular slot (of a quarter-wave length) placed roughly opposite each other, coming out onto two non-radiating opposite sides of the square, each being equipped with at least two MEMS devices, allowing the defining of at least three resonant lengths that differ according to their states. Several upper patches may also be used, each with at least one quarter-wave half-slot, with pairs of half-slots opposite each other then forming half-wave slots.
In the presence of a bridge MEMS device and rectangular slots, the bridge is preferably placed roughly parallel to the large sides of the slot. However, in the presence of a beam MEMS device and rectangular slots, said beam is preferably placed roughly perpendicularly to the large sides of the slot.
Furthermore, the lower ground plane may define a lower patch placed below the upper patch and of smaller dimensions. In this case, the module has metallic bushings that connect the ground plane to peripheral sections of the upper patch, in order to define a resonant cavity. This patch and cavity structure defines a further family of phase shifter modules.
The invention also proposes a reflectarray antenna equipped with at least two phase shifter modules of the type presented above.
The invention is particularly suited, although not exclusively, to Ku-band geostationary telecommunication antennas (12 to 18 GHz) with reconfigurable coverage (changing of orbital position, adapting of traffic), and to band C (4 to 8 GHz) or band X (8 to 12 GHz) radar antennas, and SARs in particular.
The drawings appended may not only complete the invention, but also contribute to its definition, where applicable.